Neutralization circuit for short wave transmitters



Dem 3 1, 1940. w. BUSCHBECK 2,226,694

NEUTRALIZATION CIRCUIT FOR SHORT WAVE TRANSMITTERS Filed Feb. 2, 1938OUTPUT wpu'r INPUT aurpur INVEN TOR. WEI??? BUS C HBEC' K BY A TTORNEY.

Patented Dec. 31, 1940 PATENT OFFICE NEUTRALIZATION CIRCUIT FOR. SHORTWAVE TRANSMITTERS Werner Buschbeck, Berlin, Germany, assignor toTelefunken Gesellschaft fiir Drahtlose Telegraphic in. b. H., Berli ofGermany 11, Germany, a corporation Application February 2, 1938, SerialNo. 188,233

In Germany 9 Claims.

The invention relates to a neutralizing circuit for short waves in whichessential drawbacks of the hitherto known circuits are overcome.

In order to more readily understand the invention, a discussion willfirst be given with respect to known circuit arrangements and theirdifiiculties.

, Figs. and 6 represent two well known neutralizing circuit arrangementsof the prior art.

In Fig. 5 a balance may be readily achieved between tube capacity Cagand neutralizing capacity Cn, such that a voltage introduced in theinput does not result in developing a voltage across the output.However, in this condition of adjustment, voltage across the outputcircuit sets up a potential difierence between grid and cathode by wayof the voltage dividing capacity path Cag, C1. back through theneutralizing condenser Cn because the balance before mentioned assuresthat no current fiows in the input coil system as a result of voltageacross the output circuit, and therefore there is no way for currentthrough the neutralizing condenser to influence the potential differencebetween grid and cathode. Thus, in spite of the bridge arrangement beingtheoretically perfect for preventing output voltage from reacting uponthe input circuit, it nevertheless does not prevent output voltage-frombeing fed back in degenerative phase to the input electrodes, that is,between grid and cathode, Fig. 5 is sometimes rearranged so thatthecathode of the amplifier is connected to the center of the input coilsystem rather than to the center of the capacity branch thereacross. Inthis case the same phenomenon occurs except that the amount of voltagefed back is reduced in proportion to the coefiicient of coupling betweenthe two halves of the input coil, and the phase of the feed back becomesregenerative due to the re versal of the sign of the reactance betweengrid and cathode.

In Fig. 6 a similar known neutralizing arrangement demonstrates thatwhile a perfect bridge balance will prevent any coupling between inputand output circuits, except by, virtue of the desired relay action ofthe tube, the voltage impressed upon the input circuit will produce avoltage between anode and cathode and vice versa. This again is truewhether the cathode be connected to an intermediate point on thecapacitive branch of the output circuit or tea correspondingintermediate point on the inductive branch thereof. a

Thus, it is evident that in the arrangements of This is not neutralizedby any feed March 5, 1937 (01. run-171) the known art a bridge balancingachieves only one of two desirable results. It is the object of thepresent invention to attain both the complete decoupling of input andoutput circuits other than through the electron relaying action of thetube and also to prevent voltage in the output circuit from causing anypotential difference between the tube input electrodes. This is done byemploying an auxiliary, symmetrically arranged network, arranged toprovide a resonant short circuit path at the operating frequency betweenthe cathode and the cold electrode which is connected to thedividedcircuit of the ampliher. This auxiliary circuit may, however, inpractice be coalesced with the already existing divided circuit.

The invention will now be described in greater detail in connection withthe accompanying drawings, wherein:

Figs. 1 and 2 illustrate two ways of eliminating feed-back in the gridneutralizing circuit in accordance with the invention, and

Figs. 3 and 4 illustrate two ways of eliminating feed-back in the anodeneutralizing circuit in accordance with the invention.

Figs. 5 and 6 represent well known circuit arrangements shown for thepurpose of the above exposition.

Fig. 1 shows a grid neutralizing circuit comprising the tube R, thegrid-anode capacity GAG, the neutralizing capacity Cu and the capacitiesC1 and C2 of the oscillatory circuit. Item G represents the grid point,K is the cathode point, while A designates the anode point and N theneutral point. The oscillatory input circuit lies between N and G. Inaccordance with the invention a T element is placed in parallel to thiscircuit, said T member consisting of the two inductances L and acapacity C. The inductances L may serve at the same time as the coil ofthe oscillatory circuit. Without the condenser C a reactive couplingpotential would exist at the grid point G which is determined by theproportion of the capacities CAG- and 01. This reactive coupling could,of course, be reduced at will by increasing the value of C1, but this isnot feasible in case of short waves, and ultra short waves. If thecapacity C is inserted and tuned in such a manner that the parallelconnection of the two coil halves through which the currents arrivingvia Cu and Gag pass in opposite directions, in other,

words the value (wherein M is the mutual inductance between the twohalves) is in resonance with C, a short circuit will be formed between Gand K and between N and K as viewed from the plate cathode circuit. Inother words, the feed back will be reduced to zero. The T element mayconsist either of two inductances and one capacity, as shown in Fig. 1,or it may consist of two capacities and one inductance, according toFig. 2. In applying the invention, it is wholly immaterial whether thevoltage division in the neutralizing bridge proper is capacitive orinductive.

In like manner, the passing of energy from the control transmitter tothe main transmitter can be eliminated in the anode neutralizing circuitwhile the circuit need not be further developed into a push-pull doublebridge as Was hitherto the case. The T element lies, as shown in Fig. 3,between the points A, N, and K. In Fig. 3 there is represented for thesake of clearness the bridge arrangement for the entire circuit, wherebyR is again the tube, C1 and C2 the voltage divider capacities and A, N,K the anode point, neutral point and cathode point respectively. BetweenK and G the control transmitter is inserted and between A and N the maintransmitter. At the proper tuning of the T element, the anodecathodepath is short circuited as viewed from the control transmitter. Fig. 4shows another mode of embodiment of Fig. 3 which corresponds to Fig. 1.It should, moreover, be emphasized that the efiectiveness of thearrangement described is not limited to rigorously exact seriesresonance tuning of the T element and that anywhere in the proximity ofresonance an essential improvement over the hitherto known circuits isobtained. The resultant impedance X of a series element having theresonance resistance X0 increases at first gradually as the actualfrequency w departs from the resonant frequency 200, in accordance withthe equation For obtaining substantially all the results of theinvention, it is only necessary that this value remain lower than thereactance XAG of the gridanode capacity.

The conditions are not quite as simple if the cathode, or anode, or bothelectrodes are not directly accessible, which in the case of ultra shortwaves is mostly the case owing to the electrode lead-ins. In this case,at anode neutralization, the resistance value of the T member, as viewedfrom the side of the control transmitter, must no longer be zero, but itmust be so apportioned that the voltage produced directly at theanode-cathode path by the exciting alternating voltage disappears. Thedetermination of the required resistance of the T element, by way ofcalculation, depends on the type of the circuit and after all on thetype of the tube employed. An especially simple calculation is possiblein case of tubes having a concentric grid and cathode and twin cathodestructure, since hereby a bridge relationship can be deduced in whichthe anodecathode path lies in a diagonal of the bridge containing theinductances of the electrode leadins. The actual calculation will not becarried through herein, since it does not modify the general prin-'ciple of the invention. It may be said that there lies a greateradvantage in carrying out the correct tuning of the T member throughexperimentation.

The invention can be applied in almost all bridge neutralizing circuitsin which, at exact balance, a counter coupling or feed back occursbetween an input or output circuit and a pair of tube electrodes.

What is claimed is:

1. The combination with a high frequency system having an input andanoutput circuit, and an electron discharge device having a grid and acathode connected across a portion of the input circuit, and an anodeand said cathode connected to said output circuit, said system havingmeans for preventing voltage across said output circuit trom producing apotential diiierence between the terminals of said input circuit, of areactance in the connection between said cathode and said input circuit,said reactance being of opposite sign to the reactance of that portionof the input circuit to which it is connected and having a magnitudesuch that voltage across said output circuit produces a voltage dropacross said reactance equal and opposite to the voltage across saidportion of said input circuit at the operating frequency, wherebyvoltage across said output circuit produces neither any potentialdifference across said input circuit nor any appreciable potentialdifierence between grid and cathode.

2. The combination with a highfrequency system having an input and anoutput circuit, and an electron discharge device having an anode and acathode connected across a portion of the out put circuit, anda grid andsaid cathode connected to said input circuit, said system having meansfor preventing voltage across said input circuit from producing apotential difierence between the terminals of said output circuit, of areactance in the connection between said cathode and said outputcircuit, said reactance being of opposite sign to the reactance of thatportion of the output circuit to which it is connected and having amagnitude such that voltage across said input circuit produces a voltagedrop across said reactance equal and opposite to the voltage across saidportion of said output circuit at the operating frequency, wherebyvoltage across said input circuit produces neither any potentialdiiierence across said output circuit nor any appreciable potentialdifference between anode and cathode.

3. In an amplifier circuit having a vacuum tube having input and outputelectrodes, an output circuit connected to said output electrodes, andan input circuit for energizing said input electrodes, the method ofeliminating feed-back of voltage from said output circuit to said inputelectrodes due to inherent interelectrode capacity, Which'inoludes as afirst step neutralizing voltage across said input circuit due to voltageacross saidoutput circuit, and as a second step utilizing resonanceeiiects to eliminate residual voltage between input electrodes due topotential difierence existing across the portion of said input circuitto which said input electrodes are connected.

4. In an amplifier circuit having a vacuum tube having input andoutputelectrodes, an output circuit connected to said output electrodes,and an input circuit for energizing said input electrodes, the method ofeliminating feed-back of voltage from said output circuit to said inputelectrodes due to inherent interelectrode capacity, which includes as afirst step neutralizing voltage across said input circuit due to voltageacross said output circuit, and as a second step utilizing seriesresonance to eliminate residual voltage between input electrodes due topotential difference existing across the portion of said input circuitto which said'input electrodes are connected.

5. The combination with an amplifier circuit having a vacuum tube havinga cathode and a pair of cold electrodes and input and output circuitstherefor, a bridge neutralizing circuit connected to said electrodes insuch manner that a voltage introduced in the input circuit will notresult in developing a voltage across the output, except by virtue ofthe desired relay action of the tube, said bridge neutralizing circuitincluding a divided circuit to which the cathode and one of said coldelectrodes are connected, of means providing a resonant short circuitpath at the operating (frequency between said cathode and said one coldelectrode.

6. In an amplifier circuit having a vacuum tube having input and outputelectrodes, an output circuit connected to said output electrodes, andan input circuit for energizing said input electrodes, the method ofeliminating transfer of voltage from said input circuit to said outputelectrodes due to inherent interelectrode capacity, which includes as a.first step neutralizing voltage across said input circuit due to voltageacross said output circuit, and as a second step utilizing resonanceefiects to eliminate residual voltage between output electrodes due topotential difierence existing across the portion of said output circuitto which said output electrodes are connected.

'7. In an amplifier circuit having a vacuum tube having input and outputelectrodes, an output circuit connected to said output electrodes, andan input circuit for energizing said input electrodes, the method ofeliminating transfer of voltage from said input circuit to said outputelectrodes due to inherent interelectrode capacity, which includes as afirst step neutralizing voltage across said input circuit due to voltageacross said output circuit, and as a second step utilizing seriesresonance to eliminate residual voltage between output electrodes due topotential diiTerence existing across the portion of said output circuitto which said output electrodes are connected.

8. In an amplifier circuit having a vacuum tube having first and secondsets of electrodes, an output circuit connected to said secondelectrodes and an input circuit for energizing said first electrodes,the method of eliminating transfer of voltage from one of said circuitsto that set of electrodes to which it is not directly connected due toinherent interelectrode capacity, which includes as a first stepneutralizing voltage across said input circuit due tovoltage across saidoutput circuit, and as a second step utilizing resonance efiects toeliminate residual voltage between the electrodes of said one set due topotential difierence existing across the portion of the circuit to whichsaid one set of electrodes is connected.

9. In an amplifier circuit having a vacuum tube having first and secondsets of electrodes, an output circuit connected to said secondelectrodes and an input circuit for energizing said first electrodes,the method of eliminating transfer of voltage from one of said circuitsto that set of electrodes to which it is not directly connected due toinherent interelectrode capacity, which includes as a first stepneutralizing voltage across said input circuit due to voltage acrosssaid output circuit, and as a second step utilizing series resonance toeliminate residual voltage between the electrodes of said one set due topotential difierence existing across the portion of the circuit to whichsaid one set of electrodes is connected.

WERNER BUSCI-IBECK.

